The most pertinent prior art of which applicants are aware is their copending application, U.S. Ser. No. 07/169,920, filed Mar. 18, 1988, (now U.S. Pat. No. 4,88,779 issued Dec. 19, 1989), and to the assigned to the present invention. A coding method for a constrained system using a rate 8/10 code with spectral null at f=0 is described therein. While the coding method therein described operates very satisfactorily to achieve the stated objectives, it is sensitive to phase shifts in channel output signal. As a result, an inversion of the signal polarity can cause error propagation at the output of the decoder. Also, the code on occasion generates quasi-catastrophic sequences. These sequences represent more than one distinct path through the canonical state diagrams that describe sequences with spectral null constraints. Quasi-catastrophic sequences degrade the worst case coding gain and result in decoding delays.
There is a need for a method employing a matched spectral null (MSN) trellis code that: (1) is indifferent to 180-degree phase shifts in channel output signal (an important consideration in magnetic recording, where the state of the write driver and polarity of recording head leads may be unknown); (2) eliminates all quasi-catastrophic sequences; (3) eliminates the need for differential encoding/decoding circuits typically required for application of phase invariant codes; (4) decreases encoder/decoder complexity relative to the 8/10 code described in the above-cited application; and (5) achieves the foregoing without reduction in the coding gain.